Photocontrol of Solvent Responsiveness of Structural Colored

May 23, 2013 - The color change accompanying the size change was successfully reproduced by assuming that the total amount of polymer in the thin film...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Photocontrol of Solvent Responsiveness of Structural Colored Balloons Masafumi Inoue,† Kenji Higashiguchi,*,†,‡ and Kenji Matsuda*,† †

Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡ PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan S Supporting Information *

ABSTRACT: The structural colored balloons (SCBs) composed of poly(vinyl cinnamate) (PVCi) showed solvent responsiveness of structural color by the change of shell thickness along with the size change by osmotic pressure, and the gradual color change could be stopped by UV irradiation. The rate of size change was decreased by the increase of the molecular weight upon photocross-linking reaction of PVCi.

■

INTRODUCTION Structural colors of objects are caused by optical effects such as interference, diffraction, and reflection when the size range of the objects is comparable to the optical wavelength range.1,2 The developing color strongly depends on the refractive index and the size of periodic structure. Dynamic development of structural color was achieved by using soft matter such as polymer,3−6 gels,7−11 and liquid crystals12,13 because soft matter can change the chemical entity and the periodic structure. Balloon-shaped microstructures made of polystyrene (PS) microcapsules showed structural color in water, and they are called structural colored balloons (SCBs).14 The diameters of SCBs are a few hundred micrometers, and their shell thicknesses are about a few hundred nanometers. The wavelength of the developing color can be analyzed by the shell thickness and the angle of incident light when the diameter is large excess compared with the shell thickness. In our previous work, the structural color of SCBs made of PS showed responsiveness to the solvent.15 When the surrounding solvent was transferred from water to the acetone/water mixture, the SCBs swelled initially by the influx of outer solvent due to osmotic pressure. The radius was acceleratedly increased reflecting the gradual solvation of shell film. Subsequently, shrinking was observed along with the outflow of inner solvent from the crack of the shell due to the limitation of mechanical durability. It was noted that the shell thickness could increase without folding of the polymer film during shrinking. The color change accompanying the size change was successfully reproduced by assuming that the total amount of polymer in the thin film does not change. In this study, we employed poly(vinyl cinnamate), PVCi, for the shell film in the place of PS. PVCi, which is the representative negative photoresist polymer, undergoes cyclo© XXXX American Chemical Society

addition reaction to form cyclobutane ring by irradiation with UV light between the double bonds on the same or different chains.16,17 Therefore, the molecular weight increases acceleratedly, and gel polymer forms due to the cross-linking on the multipoint. As a result, the solubility becomes lower and the mechanical property changes by UV exposure. Young’s modulus of elasticity, which means the curvature against stress, is enlarged.18 The size change of SCBs was expected to be suppressed by photo-cross-linking reaction as shown in Figure 1. Thus, the change of developing color is also expected to be suppressed.

■

EXPERIMENTAL SECTION

Materials. Poly(vinyl alcohol) (88 mol % hydrolyzed, labeled as Mw ∼ 25 000, Polysciences, Inc.), gelatin, cinnamoyl chloride, and solvents are commercially available. Preparation of PVCi. To a dry pyridine (21 mL) solution of poly(vinyl alcohol) (1.5 g, 15.9 mmol of hydroxyl groups), dry pyridine (21 mL) was added for dilution at 50 °C. A dry acetone solution of cinnamoyl chloride (7.1 g, 57 mmol) was added dropwise to prepare poly(vinyl cinnamate).17 The precipitates of PVCi were washed with water and reprecipitated as follows: a large excess of cooled methanol was added dropwise to the 1 wt % toluene solution of PVCi. There was obtained PVCi (4.4 g, 92%, pale brown powder): 1H NMR (TMS, 500 MHz, CD2Cl2) δ 1.93 (br, 2H), 5.10 (br, 1H), 6.27 (br, 1H), 7.10−7.22 (br, 5H), 7.50 (br, 1H). Elemental analysis. Found: C 74.31%, H 5.94%. Preparation of Structural-Colored Balloons. SCBs were prepared by the method of double-surface emulsion19 that is modified from the method previously reported.14,15 First, three types of solution Received: February 8, 2013 Revised: May 10, 2013

A

dx.doi.org/10.1021/la400534s | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

cinnamoyl chloride in pyridine under reflux,17 and the PSstandard Mw was analyzed as 1.77 × 105. SCBs composed of PVCi were prepared by the method of double-surface emulsion19 using an aqueous solution of gelatin and dichloromethane solution of PVCi, as described in the Experimental Section. The obtained SCBs showed several structural colors depending on shell thickness. An SCB was picked up and observed by optical microscope and SEM as shown in Figure 2. Figure 1. The concept of the photocontrol of the change of the structural color accompanying the size change of SCBs. Cross-linking reaction occurred by UV irradiation to SCB composed by PVCi; thus, the size change is expected to be suppressed, and the color became invariant. The gray color means the curing of PVCi upon cross-linking. In this figure, the preceding swelling was omitted for clarity. The difference of the light path 2d shown as bold line has been discussed in further detail in the literature.14 L and D are the radius and thickness of the shell, respectively. The subscript i indicates the initial state.

Figure 2. (a) Optical micrograph of the SCB at 20× magnification under unpolarized transmitted light. (b) SEM image of the dried SCB. (c) Cross section of SCB shell of (b).

were prepared as follows: (1) 100 mg of gelatin was dissolved in 10 mL of water at 80 °C and cooled to room temperature to form gelatin aqueous solution α; (2) 60 mg of PVCi was dissolved in 3.0 mL of dichloromethane to produce a 2 wt % PVCi solution, namely, PVCi organic solution β; (3) 1.5 g of gelatin was dissolved in 150 mL of water at 40 °C and cooled to room temperature to produce a 1 wt % solution, namely, gelatin aqueous solution γ. The 3 mL gelatin aqueous solution α was added to polystyrene organic solution β and stirred vigorously (1350 rpm) to generate an oil/water emulsion (first emulsification). Subsequently, the first emulsion was poured into the gelatin aqueous solution γ with stirring at 500 rpm, and water/oil/water double-surface emulsion was obtained (second emulsification). It should be noted that mixing was first carried out at room temperature, and then, the temperature was gradually increased to 40 °C over 4 h to remove dichloromethane completely. After the mixture was cooled to room temperature, the dispersion was washed many times with pure water, so that PVCi microcapsules were obtained as structural-colored balloons with different sizes. All the balloons were purified using appropriate sieves. SEM. Scanning electron microscopy was carried out using a Hitachi S-4700 to obtain a 3D image of the SCB cross section. The dried SCB under low pressure was Au-coated under vacuum using Sanyu ion spatter SC-70f. The deposited metal thickness is 20 nm. The observation was performed under 5 kV accelerating voltage. Optical Microscope. Using a Nikon SMZ-2B stereomicroscope, some SCBs were chosen and moved to flow cell whose thickness is 5.0 mm. Acetone/water was premixed and the solvent was introduced using KdScientific KDS100 syringe pump at 0.17 mL/min. Direct observation of the size and color change of SCB was carried out in transmission geometry using Nikon ECLIPSE LV100 coupled with Nikon DS-Fi1 CCD camera. The objective lenses were Nikon Plan-Fluor 10×/0.30 and 20×/0.50. UV irradiation to a selected SCB before solvent transform was carried out using epifluorescence microscope with 130 W mercury lamp (G-HGFI, Nikon) and filter set (interference filter FF01-341/LP, dichroic mirror FF409-Di02, and interference filter FF01-390/40, Semrock) for photo-cross-linking reaction of PVCi. UV irradiation in the course of solvent substitution was performed by use of 200 W Hg−Xe lamp guided by optical fiber isolated by passing the light through a combination of long-pass and band-pass filters (UV-29 and UV-D33S, AGC Techno Glass). The power was measured by OPHIR 3A-FS as 11.7 mW/cm2. The luminescent spectra to irradiate SBC were measured by using a miniature fiber-optic spectrometer Ocean Optics S2000 with a 600 μm optical fiber as shown in Figure S3.

Because the refractive index of PVCi (n = 1.61)20,21 is comparable to the PS (n = 1.59),22 the optical characteristics of SCBs composed of these polymers were similar. Additionally, the solvation property to the mixed solvent of acetone/water is also similar between both polymers; therefore, the solvent responsiveness was also similar, e.g., SCBs composed of PVCi showed swelling and shrinking. Photo-Cross-Linking of PVCi. Photo-cross-linking was carried out by focused UV light using low output mercury lamp (Figure S3a) under the epi-illuminated microscope (Figure 3).

Figure 3. (a) Micrograph of SCBs in a flow cell before photoirradiation and exchange of solvent. A hexagon written by broken purple lines shows the region of UV irradiation. The expanded images showed the time course of size and color changing of (b) unirradiated and (c) UV-irradiated SCBs which corresponds to the black and violet arrows in (a), respectively. (d) Changes in diameter, (L − Li)/Li, for unirradiated (open circles) and UV-irradiated (violet solid squares) SCBs. The ratio of the injected acetone/water was 4/6.

Irradiation was performed on the selected SCB before exchanging the surrounding water of SCB. Although the refractive index of shell changed slightly because of the loss of double bond and the change of free volume by formation of covalent bond, it was not enough to change the structural color of the SCB. At 15 min after the photoirradiation, the surrounding solvent of SCBs was exchanged from water to

■

RESULTS AND DISCUSSION Preparation of SCBs Composed of PVCi. PVCi was synthesized by mixing purchased poly(vinyl alcohol) and B

dx.doi.org/10.1021/la400534s | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

system, the photocontrol could not be achieved. It was suspected that the distance between double bonds and the motion of segments increased in the solvated PVCi. In this experiment, the shell thickness was not experimentally measured by use of SEM but could be determined as a unique solution of the simulation of size change. The estimation is possible when the range of the color change is wide. The candidates of initial thickness Di were 310−320 nm (m = 0), 580−640 nm (m = 1), 850−950 nm (m = 2−3), and 1100− 1250 nm (m = 3−4) considering the observed reddish-orange color at 0 min and the simulated interference color. Assuming that the amount of shell polymer did not change, the film thickness D corresponding to each radius L was calculated. The developing color for each thickness was then calculated and compared with the observed color. In this case, the initial thickness Di was suspected as 610 ± 10 nm (m = 1) and the initial reflection orders of m = 0, 2−3, and 3−4 were excluded because the calculated color change from shell thickness cannot reproduce the observed color change (Figure 5).

the acetone/water mixture with a ratio of 4/6. SCBs that were not irradiated were slightly swelled at first and then shrunk considerably. Figure 3b shows the structural color of an SCB changed from green to peacock green via red while the diameter changed from 180 to 132 μm. On the other hand, UV-irradiated SCB, which had diameter of 182 μm, comparable to the unirradiated SCB, rapidly swelled at first but hardly shrunk; thus, the color changed from greenish blue to blue at the rapid swelling, but during shrinking the structural color scarcely changed as shown in Figure 3c. Here (L − Li)/Li was employed for the ratio of size change to correct the size of SCBs having different initial diameter (Figure 3d). At 19 h after photoirradiation, the ratio of size change of unirradiated and irradiated SCBs were −25 and +3%, respectively. The rate of shrinking quite depends on the molecular weight of polymer shell when the ratio of acetone/water was constant.15 Thus, the shrinking rate became slow. The turning point from swelling to shrinking shifted to later time than the unirradiated SCBs. The initial swelling continued until the mechanical durability of PVCi film; thus, the UVirradiated SCB was hard to break because of the increase of molecular weight and cross-linking points. Because the formation of cracks became difficult by the high mechanical durability, some SCBs did not shrink (Figure S4). Photocontrol of Solvent Responsiveness. To achieve the stopping of color change caused by the size change of SCB, photo-cross-linking was employed. Figure 4 shows the SCB to

Figure 5. Changes in diameter (violet solid squares) and the expected shell thickness (violet open squares) of the SCB which was described in Figure 4. The simulated interference color considering multiple interference15 and the magnified images were shown next to the y-axis using the expected initial shell thickness Di = 610 nm. The ratio of acetone/water was 5/5.

Characterization of Photoreacted PVCi. The photoreaction of PVCi was traced by GPC. The increase of molecular weight was observed (Figure S2). UV irradiation was carried out to a THF solution of PVCi. In the case of cast-coated PVCi, the film became insoluble to acetone upon UV irradiation. The photoreaction of thin film on glass was carried out using epiillumination system and analyzed by IR spectroscopy as shown in Figure 6. The photogeneration of cyclobutane was confirmed by the decrease of CC stretching vibration (1640 cm−1) and π-conjugated CO stretching vibration (1710 cm−1) and the increase of unconjugated CO stretching vibration (1740 cm−1). It was confirmed that the cross-linking reaction of PVCi efficiently occurs by UV irradiation, and consequently the molecular weight increases. The solvent responsiveness is very sensitive to the molecular weight; the threshold molecular weight presents between 3.2 × 105 and 4.3 × 105 in the case of PS with the ratio of acetone/water = 4/6 as shown in our previous paper.15 The measurement of molecular weight before and after photoirradiation suggests that the change across the threshold value occurred in the shell of SCBs. The solvation of polymer film is considered to affect strongly the rate of size change of SCBs because the shrinking occurs with squeezing of inner solvent by surface tension of cracked SCBs. The decrease of permeability is considered to be due to the increase of molecular weight. In our previous work,15 the

Figure 4. (a) Micrograph of SCBs under exchanging of surrounding solvent the time course of size and color changing of UV-irradiated SCBs. (b) Changes in diameter, (L − Li)/Li, of unirradiated (open circles) and UV-irradiated (violet solid squares) SCBs. UV irradiation was carried out for 10 min from 127 to 137 min. The ratio of acetone/ water was 5/5.

which UV irradiation was carried out during shrinking as illustrated in Figure 1. At first, the surrounding solvent was exchanged from water to the acetone/water mixture with a ratio of 5/5. For easy observation, the ratio of acetone was increased to enhance the rate of size change. Figure 4b shows the detailed size change of the UV-irradiated and unirradiated SCBs; both SCBs were slightly swelled likewise the above-mentioned one because UV light was not irradiated at this period. The turning points from swelling to shrinking were slightly different because each SCB has different defects of film. In the case without photoirradiation, the SCB gradually shrunk continuously to −38%. On the other hand, when UV irradiation was carried out to another SCB for 10 min during shrinking from 127 to 137 min, by use of high-output mercury lamp (Figure S3b), the size change suddenly stopped at −15% and the blue-green color was held. In contrast, in the case using low-output epi-illuminated C

dx.doi.org/10.1021/la400534s | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

(2) Cha, Y.; Tsunooka, M.; Tanaka, M. Preparation and Optical Properties of Polymer Films Having Lamellar Structure and Cylindrical Air Bubbles. Angew. Makromol. Chem. 1985, 129, 155−161. (3) Ishizu, K.; Yasuda, M.; Sato, Y.; Tamura, T. Solvent-Induced Reversible Color Changes in Block Copolymer Films and New Locking Method of Structural Colors. Polym. Adv. Technol. 2005, 16, 628−632. (4) Finlayson, C. E.; Spahn, P.; Snoswell, D. R. E.; Yates, G.; Kontogeorgos, A.; Haines, A. I.; Hellmann, G. P.; Baumberg, J. J. 3D Bulk Ordering in Macroscopic Solid Opaline Films by Edge-Induced Rotational Shearing. Adv. Mater. 2011, 23, 1540−1544. (5) Holtz, J. H.; Asher, S. A. Polymerized Colloidal Crystal Hydrogel Films as Intelligent Chemical Sensing Materials. Nature 1997, 389, 829−832. (6) Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. PhotonicCrystal Full-Colour Displays. Nat. Photonics 2007, 1, 468−472. (7) Hu, Z.; Huang, G. A New Route to Crystalline Hydrogels, Guided by a Phase Diagram. Angew. Chem., Int. Ed. 2003, 42, 4799− 4802. (8) Fudouzi, H.; Xia, Y. Colloidal Crystals with Tunable Colors and Their Use as Photonic Papers. Langmuir 2003, 19, 9653−9660. (9) Harun-Ur-Rashid, M.; Seki, T.; Takeoka, Y. Structural Colored Gels for Tunable Soft Photonic Crystals. Chem. Rec. 2009, 9, 87−105. (10) Takeoka, Y. Structural Colored Gel. J. Photopolym. Sci. Technol. 2009, 22, 123−132. (11) Hu, Z.; Lu, X.; Gao, J. Hydrogel Opals. Adv. Mater. 2001, 13, 1708−1712. (12) Kikuchi, H.; Yokota, M.; Hisakado, Y.; Yang, H.; Kajiyama, T. Polymer-Stabilized Liquid Crystal Blue Phases. Nat. Mater. 2002, 1, 64−68. (13) Abraham, S.; Mallia, V. A.; Ratheesh, K. V.; Tamaoki, N.; Das, S. Reversible Thermal and Photochemical Switching of Liquid Crystalline Phases and Luminescence in Diphenylbutadiene-Based Mesogenic Dimers. J. Am. Chem. Soc. 2006, 128, 7692−7698. (14) Ikkai, F. Structural Colored Balloons Consisting of Polystyrene Microcapsules in Water. Langmuir 2008, 24, 3412−3416. (15) Higashiguchi, K.; Inoue, M.; Oda, T.; Matsuda, K. SolventResponsive Structural Colored Balloons. Langmuir 2012, 28, 5432− 5437. (16) Xiaoming, D.; Jinbao, G.; Yang, J.; Jie, W. Light-Control Birefringence of Oriented Poly(Vinyl Cinnamate) by UV Irradiation. J. Appl. Polym. Sci. 2010, 116, 3367−3372. (17) Minsk, L. M.; Smith, J. G.; Van Deusen, W. P.; Wright, J. F. Photosensitive Polymers. I. Cinnamate Esters of Poly(Vinyl Alcohol) and Cellulose. J. Appl. Polym. Sci. 1959, 6, 302−307. (18) Haramina, T.; Kirchheim, R. Mechanical Spectroscopy of PVCN with Increasing Cross-Linking Degree. Macromolecules 2007, 40, 4211−4216. (19) Mora-Huertas, C. E.; Fessi, H.; Elaissari, A. Polymer-Based Nanocapsules for Drug Delivery. Int. J. Pharm. 2010, 385, 113−142. (20) Imane, A.; Dominique, B.; Isabelle, H. Improvements of the Poly(Vinyl Cinnamate) Photoresponse in Order to Induce High Refractive Index Variations. J. Phys. Chem. B 2004, 108, 2801−2806. (21) Dominique, B.; Philippe, G.; Isabelle, H.; Imane, A.; Thomas, B.; Severine, H.; Bruno, V. High Refractive Index Contrast in a Photosensitive Polymer and Waveguide Photo-Printing Demonstration. Opt. Commun. 2004, 235, 281−284. (22) Ma, X.; Q Lu, J.; Brock, R. S.; Jacobs, K. M.; Yang, P.; Hu, X.-H. Determination of Complex Refractive Index of Polystyrene Microspheres from 370 to 1610 nm. Phys. Med. Biol. 2003, 48, 4165−4172. (23) Kaliappan, S. K.; Cappella, B. Temperature Dependent ElasticPlastic Behaviour of Polystyrene Studied Using AFM Force-Distance Curves. Polymer 2005, 46, 11416−11423. (24) Amy, R. C.; Rodney, A. D.; Gaylen, M. Z. Water Permeability in Poly(Ortho Ester)s. J. Membr. Sci. 1992, 65, 269−275. (25) Rong, H. C.; Homg-Dar, H. Effect of Molecular Weight of Chitosan with the Same Degree of Deacetylation on the Thermal, Mechanical, and Permeability Properties of the Prepared Membrane. Carbohydr. Polym. 1996, 29, 353−358.

Figure 6. (a) IR spectral change by irradiation with UV light under epi-illumination of optical microscope to thin film of PVCi on glass and (b) the expanded spectra around the remarkable peaks for the cross-linking reaction; CC stretching, π-conjugated CO stretching, and unconjugated CO stretching vibration appeared at 1640, 1710, and 1740 cm−1, respectively.

rate of size change has significant dependence of Mw when linear PS was employed and the threshold of Mw appears at a narrow range between 3.2 × 105 and 4.3 × 105 as described above. But the Young’s modulus of linear PS scarcely changes from 3 to 3.5 GPa when Mn changes from 0.42 × 105 to 6.2 × 105 as shown in the literature.23 Additionally, cross-linking are reported to affect the permeability of the film.24,25 The reason is that solvent molecule hardly permeates26 small network of cross-linked polymer27,28 and that cross-linking decreases the diffusion channel.29 Additionally, mechanical durability increases by formation of the 3D network,30−32 and therefore the deformation of shell film gets difficult. Therefore, the stopping of size and color change was successfully performed by crosslinking.

■

CONCLUSIONS The structural colored balloons (SCBs) showed solvent responsiveness of structural color by the change of shell thickness along with the size change by osmotic pressure. When poly(vinyl cinnamate) was used for SCBs, the gradual color change could be stopped by UV irradiation. The rate of size change was dependent on the molecular weight which increases by photo-cross-linking reaction of PVCi.

■

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.M.); higashi@ sbchem.kyoto-u.ac.jp (K.H.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (No. 23107535) on the Innovative Areas “Fusion Materials” (Area no. 2206) from MEXT and PRESTO, JST.

■

REFERENCES

(1) Khudiyev, T.; Ozgur, E.; Yaman, M.; Bayindir, M. Structural Coloring in Large Scale Core−Shell Nanowires. Nano Lett. 2011, 11, 4661−4665. D

dx.doi.org/10.1021/la400534s | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

(26) Yeon, C. K.; Lee, K. H. A Study on Permeation Behavior of a Liquid Mixture through PVA Membranes Having a Crosslinking Gradient Structure in Pervaporation. J. Appl. Polym. Sci. 1996, 59, 1271−1279. (27) Christopher, S. B.; Nikolaos, A. P. Mechanisms of Solute and Drug Transport in Relaxing Swellable Hydrophilic Glassy Polymers. Polymer 1999, 40, 3383−3398. (28) Berger, J.; Reist, M.; Mayer, J. M.; Felt, O.; Peppas, N. A.; Gurny, R. Structure and Interactions in Covalently and Ionically Crosslinked Chitosan Hydrogels for Biomedical Applications. Eur. J. Pharm. Biopharm. 2004, 57, 19−34. (29) Ichikawa, H.; Tokumitsu, H.; Jono, K.; Fukuda, T.; Osako, Y.; Fukumori, Y. Coating of Pharmaceutical Powders by Fluidized Bed Process. VI. Microencapsulation Using Blend and Composite Latices of Copoly(EA-MMA-HEMA). Chem. Pharm. Bull. 1994, 42, 1308− 1314. (30) Urlam, M. K.; Adam, W. M.; Jeffrey, M. C.; Elliot, L. C. Hydrazone Self-Crosslinking of Multiphase Elastin-Like Block Copolymer Networks. Acta Biomater. 2012, 8, 988−997. (31) Jad, A. J.; Joseph, B. S. Mechanical Properties of Reversibly Cross-Linked Ultrathin Polyelectrolyte Complexes. J. Am. Chem. Soc. 2006, 128, 2940−2947. (32) Fwu, L. M.; Chih, Y. K.; Shin, S. S.; Sung, T. L.; Shon, F. C. The Study of Gelation Kinetics and Chain-Relaxation Properties of Glutaraldehyde-Cross-Linked Chitosan Gel and Their Effects on Microspheres Preparation and Drug Release. Carbohydr. Polym. 2000, 41, 389−396.

E

dx.doi.org/10.1021/la400534s | Langmuir XXXX, XXX, XXX−XXX